As known, a yarn feeder for textile/knitting lines typically comprises a stationary drum on which a motorized flywheel winds a plurality of yarn loops forming a well reserve. Upon request from a dowstream machine, e.g., a loom, the loops are unwound from the drum and, before reaching the machine, the yarn passes through a weft-braking device that influences the tension of the unwinding yarn.
WO91/14032 discloses a weft-braking device, in which the yarn unwinding from the drum radially runs between two counterposed annular plates which are coaxially arranged in front of the drum and are biased against each other in order to brake the yarn by friction. The plate facing the drum is stationary, while the opposite plate is biased against the stationary plate by a spring or an actuator, e.g., an electromagnetic actuator which is controlled in order to adjust the amount of braking action applied to the yarn.
In WO02/22483, the unwinding yarn also runs radially between two counterposed annular plates which are coaxially arranged in front of the drum. The plate facing the drum is made of a magnetic material and is axially slidable on a pin. The opposite plate is stationary and has an electromagnet arranged behind it which, when energized, attracts the movable plate against the stationary plate, whereby a braking action is applied to the yarn which depends on the current across the electromagnet.
An advantage of the above-mentioned braking systems is that they do not require frequent cleaning operations because the dust and paraffine generated by the yarn running between the braking surfaces are swept away by the swivel movement of the yarn unwinding from the drum.
However, the above-mentioned systems also have some drawbacks.
In particular, the electromagnetically operated braking systems, which are widespread nowadays, are not entirely satisfactory in terms of reaction times. Particularly, it is known that the excitation times of the coils are not negligible; in addition, in the case of WO02/22483, the movable plate made of a magnetic material has a relatively heavyweight structure, resulting in a considerable inertia which further slows down the reactivity of the system.
In addition, it is also known that the electromagnetically operated weft-braking devices require high currents and, consequently, high power, with consequent disadvantages in terms of energy consumption, especially in view of the fact that a conventional textile/knitting line often makes use of dozens of feeders for a single downstream machine.